What keeps a cell’s contents away from its environment?

Mainly the cell/plasma membrane controls the import or export of materials, thus also maintaining that intracellular (inside of the cell) structures stay protected from the extracellular (outside) environment.

░░░░░░░░░░░░▄▐░░░░░░ ░░░░░░▄▄▄░░▄██▄░░░░░ ░░░░░▐▀█▀▌░░░░▀█▄░░░ ░░░░░▐█▄█▌░░░░░░▀█▄░ ░░░░░░▀▄▀░░░▄▄▄▄▄▀▀░ ░░░░▄▄▄██▀▀▀▀░░░░░░░ ░░░█▀▄▄▄█░▀▀░░░░░░░░ ░░░▌░▄▄▄▐▌▀▀▀░░░░░░░ ▄░▐░░░▄▄░█░▀▀░░░░░░░ u have been spooked by the ▀█▌░░░▄░▀█▀░▀░░░░░░░ ░░░░░░░▄▄▐▌▄▄░░░░░░░ ░░░░░░░▀███▀█░▄░░░░░ ░░░░░░▐▌▀▄▀▄▀▐▄░░░░░ spooky skilenton ░░░░░▐▀░░░░░░▐▌░░░░░ ░░░░░░█░░░░░░░░█░░░░ ░░░░░▐▌░░░░░░░░░█░░░ ░░░░░█░░░░░░░░░░▐▌░░ send this to 4 ppl or skelintons will eat you